litewitness: Document how to think about database backups

Doesn't have to be long, but this question will surely come up very soon.

The issue names litewitness but no file, test, or documentation location. Start by locating its existing documentation and database-related entry point, then outline how backups should be considered and what users need to know. Done means the backup guidance is documented clearly enough to answer the expected question without being long.

Written by the indexing model from the issue text.